The invention relates to a low loss method and device for modifying the flow from a constant rate feed pump by chopping the flow into one fraction supplied as a power flow fraction to a load and into a further fraction returned under zero pressure.
The oil requirement arising in closed circuit hydrostatic systems is greatly dependent on the different settings of the equipment and on leakages, a fact that so far has been taken into account by the selection of a particularly large feed pump which is able to meet the maximum requirement of the load at a given feed pressure. In such a system, which is elaborate and expensive, the oil supply occurring in some phases which is not able to be used, is returned to the tank via a pressure limiting valve. This amounts to power dissipation leading to the production of heat in the system.
Another possibility which has heretofore been used is an arrangement with a number of coupled feed pumps corresponding to different supply rates which are able to be called upon as required. Although this arrangement is more economical as regards coping with consumption, it is more expensive than the first system and furthermore requires a pressure limiting valve.
A more elegant system is one in which the feed pump is an adjustable one, but however the costs are then very high and the overall size is large.
A proposal has been made in the German patent application No. P 23 07 351 in which the pump output flow is chopped up into separate portions of liquid, which are passed alternately into respective supply ducts, the pump output flow being chopped by way of a directional control valve in a system including a pulse generator whose pulses are preferably static and have steep flanks, the flow being controlled by varying the time pulse sequence. This method is based on an externally set chopping of the pump output flow by using a pulse geerator used as a clock and which performs a periodic apportionment of the pump output flow in accordance with criteria which are permanently established, such as the frequency and the amount of the fraction, without taking into account the instantaneous supply rate to the load. There is thus an adaptation in arbitrary dictated periods of time and thus no optimum adaptation of the fractions supplied by the constant rate pump to the actual requirement of the load. A further disadvantage of this system is that a valve spool selecting the various liquid output ports is acted upon by an operating piston, whose stroke is terminally limited by striking against the front and rear walls of its pressure space, which leads to an unacceptable level of staccato clicking noise and to a high wear rate on the operating members. Apart from this, the known control system requires one pulse generator, two solenoid valves and a pressure limiting valve so that it is high in price and furthermore has a substantial space requirement.